Madrid, Spain (CNN) -- Protests against Spain's economic crisis took a new turn Wednesday as social media networks fueled calls for demonstrators to take to the streets before local elections a few days away.

Thousands returned late Tuesday to Madrid's central Puerta del Sol plaza -- where the main protests began Sunday.

A few hundred demonstrators camped out there overnight, while similar but smaller protests were held in Barcelona and other Spanish cities, a protest organizer said.

"The economy and unemployment are key to the protest because that binds all of us together," said Jon Aguirre Such, a spokesman for the Real Democracy Now, one of many groups convening the demonstrations.

"In this crisis, while some have gotten rich, most people have less income," Aguirre said.

Demonstrators are protesting Spain's 21% unemployment rate and a record 4.9 million jobless.

Protesters say a plethora of temporary labor contracts offer few or no job benefits. In addition, some are protesting against the political and financial establishment that they say is to blame.

The protests come in the closing days of the campaign for local elections set for Sunday. Spain's 8,000 cities and towns will elect mayors, along with 13 (out of 17) regional presidents and parliaments.

Zapatero, squeezed by the nation's prolonged economic crisis, announced last month he won't seek a third term in national elections due by March 2012.

For the past few years, as Spain's rate of unemployment remained tenaciously high, most of the protests were organized by major trade unions.

But the latest protests appear to be grass-roots movements fueled by social networks, sparking more attention.

Elena Ortega, who says she's managed to find only a part-time secretarial job, helped spread the word on Facebook about the protests on Wednesday.

"If this is happening, it's because the unions weren't doing what was needed, when it was needed. They haven't delivered," she said.

She said she's worried about her 20-year-old son, who has only found temporary jobs in the past four years.

"Forty percent of our young people are unemployed and don't have a chance," Ortega said.

The movement does not appear to be linked to the unions or political parties, the traditional protest heavyweights in Spain, said Economist Fernando Fernandez of the IE Business School in Madrid.

"I think we really don't know what we're seeing," Fernandez said. "This is the very beginning of a new movement. I don't expect it to become a very large social response or protest against the unemployment perspective in Spain."

But hours later, on Tuesday night, several thousand protesters returned to Madrid's central plaza. By Wednesday morning, dozens remained in their overnight encampment.

The social media networks call for renewed protests on Wednesday evening in Madrid and three dozen other Spanish cities. The aim is to continue the protests at least through the elections on Sunday, Aguirre said.

But Wednesday afternoon, Madrid's elections board banned the planned demonstration at 8 p.m. (2 p.m. ET) at the Puerta del Sol plaza. The board said there were not "extraordinary and serious reasons" to allow the demonstration on short notice, according to the central government's main regional office in Madrid.

The election board vetoed the demonstration -- which would have come during the final days of the election campaign -- on the grounds it could affect the right of citizens to vote freely, a regional office spokesman said.

Organizers of many large demonstrations in Madrid seek and receive prior authorization, usually days or weeks ahead of the event. But the economic protests since Sunday have sprung up on short notice.

Spain's El Pais newspaper reported that authorities planned to have sufficient police officers on hand to prevent the demonstration.